Field of the Invention
This invention relates generally to a mixing circuit for mixing sampled signals and, more particularly, to a mixing circuit for mixing a plurality of sampled outputs that differ in phase by 2.pi./N from one another.
An image sensor consisting of a solid-state image sensor element, such as a charge coupled device (CCD), typically employs a scattered picture element structure. In such scattered picture element image sensor, the output signals corresponding to individual picture elements are first sampled before being processed to obtain the information contained therein. For example, a luminance signal can be formed from a color image output signal obtained from a solid-state color image sensor that is provided with a color coding filter, and the color signals are obtained by performing color separation on the color image signal that can be converted through a one horizontal scanning period (1H) delay circuit into a number of simultaneous signals, which are additively synthesized in predetermined proportions to obtain the luminance signal.
In the case of a color television signal of the NTSC (National Television System Committee) system, the luminance signal Y is defined by an equation EQU Y=0.11B+0.59G+0.30R (1).
That is, the luminance signal Y can be obtained by additively synthesizing blue (B), green (G), and red (R) signals in the proportions of 0.11:0.59:0.30, respectively. Where luminance signal Y is formed from the color image output signal from a solid-state image sensor having a scattered picture element structure, it is known to remove the so-called turn-around distortion of side bands caused by sampling by forming a low frequency luminance component Y.sub.L given as EQU Y.sub.L =0.11B+0.59G+0.30R (2)
and high-frequency luminance component Y.sub.H given as EQU Y.sub.H =0.5B+G+0.5R (3)
and combining these components Y.sub.L and Y.sub.H to obtain the luminance signal Y. For example, with a one-chip solid-state color image sensor that uses a mosaic color coding filter C.sub.F, as shown in FIG. 1, having green filters G.sub.F arranged in a checkerboard-like fashion and red and blue filters R.sub.F and B.sub.F arranged in alternate horizontal lines between adjacent green filters G.sub.F. A television signal according to the NTSC system and the above-described system can be formed with a signal processing circuit having a construction as shown in FIG. 2.
In FIG. 2, a solid-state color image sensor 1 is formed as an interline transfer type CCD provided with the color coding filter C.sub.F, as described above. Solid-state color image sensor 1 provides a color image output signal at a clock frequency of 2f.sub.c. The color image output signal from image sensor 1 is coupled through buffer amplifier 2 to color separating circuit 3. Color separating circuit 3 consists of two sample/hold circuits 3A and 3B that are operated under the control of respective sampling clocks (not shown), which have a frequency f.sub.c and are 180 degrees, that is, .pi. radians, out of phase with each other. Sample/hold circuit 3A samples and holds the green signal G in the color image output signal from buffer amplifier 2. Another sample/hold circuit 3B samples and holds a red/blue signal R/B. A white balance control circuit 4 includes a fixed gain amplifier 4G for amplifying the green (G) signal, a variable gain amplifier 4R for amplifying the red (R) signal, and a variable gain amplifier 4B for amplifying the blue (B) signal. The white balance control is achieved by adjusting the gains of the variable gain amplifiers 4R and 4B, respectively, such that the red (R), green (G), and blue (B) signals are at an equal level. The green (G) and red/blue (R/B) signals, having been level adjusted in white balance control circuit 4, are coupled through respective gamma correction circuits 5A and 5B to a luminance signal forming circuit 10. Luminance signal forming circuit 10 includes first signal processing circuit 11 for effecting additive synthesis of its input signals in accordance with equation (2), second signal processing circuit 12 for effecting additive synthesis of its input signals in accordance with equation 3, low-pass filter 13 for receiving the synthesized output signal of first signal processing circuit 11, band-pass filter 14 for receiving the synthesized output signal of second signal processing circuit 12, and signal adder 15 for effecting additive syntheses of a low-frequency luminance signal Y.sub.L obtained from low-pass filter 13 and a high-frequency luminance signal Y.sub.H obtained from bandpass filter 14. Circuit 10 produces a luminance signal Y that is given by the equation Y=Y.sub.L +Y.sub.H. The three primary color signals red (R), green (G), and blue (B) having been gamma corrected in the conventional manner in gamma correction circuits 5A and 5B and the luminance signal Y formed by luminance signal synthesizing circuit 10 are supplied to color coder 8. Color coder 8 forms a composite video signal made up of the luminance signal, the green (G) signal, and the red/blue (R/B) signal in accordance with the NTSC system.
In luminance signal forming circuit 10, described above, second signal processing circuit 12 for forming the high-frequency luminance signal Y.sub.H, has a construction as shown in FIG. 3. In signal processing circuit 20 shown in FIG. 3, the green (G) and red/blue (R/B) signals obtained through color separation of the color image signal from solid-state color image sensor 1 are converted to simultaneous signals through four delay circuits 32, 34, 42, and 44 each providing a delay time equal to one horizontal scanning period 1H. These simultaneous signals are weighted and added in first and second signal adders 51 and 52 to produce addition output signals Y.sub.H1 and Y.sub.H2, respectively. The addition output signals Y.sub.H1 and Y.sub.H2 are added together in third signal adder 53 to form the high-frequency luminance signal Y.sub.H. The green (G) signal is supplied to a first signal input terminal 21 connected through first sample/hold circuit 31 to first 1H delay circuit 32 providing a one horizontal scanning period (1H) time delay. First 1H delay circuit 32 is connected through second sample/hold circuit 33 to second 1H delay circuit 34 also providing a time delay of one horizontal scanning period (1H). Second 1H delay circuit 34 is connected to third sample/hold circuit 35 providing a time delay of one horizontal scanning period. The red/blue (R/B) signal is supplied to second input signal terminal 22. As in the line for the green (G) signal described above, second signal input terminal 22 is connected through first sample/hold circuit 41, first 1H delay circuit 42 providing a time delay of one horizontal scanning period, second sample/hold circuit 43, second 1H delay circuit 44 providing a time delay of one horizontal period, and third sample/hold circuit 45. The output signals (R/B).sub.0 and (R/B).sub.2 from respective first and third sample/hold circuits 41 and 45 in the red/blue (R/B) signal line and the output signal G.sub.1 from second sample/hold circuit 33 in the green (G) signal line are coupled through respective weighting circuits 46, 48, and 37 where they are weighted in proportions of 1(R/B)H0.sub.Y :1(R/B).sub.2 :2G.sub.1. The weighted in proportions of 1 (R/B)H0.sub.Y :1 (R/B).sub.2 :2 G.sub.1. The weighted signals in these proportions are fed to a first signal adder 51. The output signals G.sub.0 and G.sub.2 from first and third sample/hold circuits 31 and 35 in the green (G) signal line and the output (R/B).sub.2 from second sample/hold circuit 43 in the red/blue (R/B) signal line are weighted in respective weighting circuits 36, 38, and 47 in the proportions of 1G.sub.0 :1G.sub.2 :2(R/B).sub.1. These weighted signals are fed to second adder 52.
The sampling and holding operations of first and third sample/hold circuits 41 and 45 in the red/blue (R/B) signal line and second sample/hold circuit 33 in the green (G) signal line are done under the control of a clock signal .phi..sub.1. The sampling and holding operation of second sample/hold circuit 43 of the red/blue (R/B) signal line and first and third sample/hold circuits 31 and 35 in the green (G) signal line are done under the control of a second clock signal .phi..sub.2 that is 180 degrees, that is, radians, out of phase with the first clock signal .phi..sub.2.
In signal processing circuit 20 of FIG. 3, sample/hold circuits 31, 33, and 35 and sample/hold circuits 41, 43, and 45 are controlled for zero hold by the respective clock signals .phi..sub.1 and .phi..sub.2 that are 180 degrees out of phase with each other, the transfer function H.sub.0 (.omega.) having a low-pass filter effect is expressed by an equation, ##EQU1## where f.sub.c is the clock frequency and T.sub.c is the period, is applied to the first and second signal adders 51 and 52. This transfer function H.sub.0 (.omega.) has a frequency characteristic shown by the dashed curve in FIG. 4.
If the luminance signal Y is to be obtained from a frequency band using a sampling rate exceeding the Nyquist criteria, as in an image sensor apparatus making use of a solid-state color image sensor, the value of the transfer function H.sub.0 (.omega.) equals 0.9000 for f=f.sub.c /4' but it is reduced to 0.637 for f=f.sub.c /2. For this reason, if signals Y.sub.H1 and Y.sub.H2 are added in the third signal adder 53, as noted above, deterioration of the frequency characteristic for a range of high-frequency values of the luminance signal Y results in reduction of the horizontal resolution, which is highly undesirable.